This invention relates to a method of detecting a marker within a predetermined zone and to a system for carrying out the method. The invention is intended primarily to be used in the detection of goods in electronic article surveillance or anti-theft systems, but it may be used for example in article tracking or personnel detection systems.
The invention concerns the detection of markers which have specific non-linear characteristics. It is exemplified in relation to high permeability ferromagnetic markers, but it applies also to markers which have non-linear electronic circuit components.
Systems which are examples of this invention will provide for the excitation and interrogation of (receipt of information from) special markers, and the systems give better distinguishability in detection of these markers over commonplace `false alarm` objects at minimum system complexity and cost, when compared to systems of the prior art. This leads to high positive detection probability and low false alarm probability.
The types of markers detected by these systems are well known in the prior art. They are usually ferromagnetic markers which have a very high magnetic permeability and low coercivity. This means that they exhibit magnetic saturation (and particularly a reproducible non-linear magnetic response) at very low levels of applied magnetic field (typically of order 1 Oersted). They are typically long narrow strips or thin films of special high permeability magnetic alloys.
In systems which detect these markers, an interrogating magnetic field is driven by a coil or set of coils. This varying magnetic field produces a varying state of magnetization in the marker which in turn re-emits a magnetic field. Because of the non-linearity of the marker, the re-emitted field contains frequency components such as harmonics and intermodulation products which are not present in the interrogating field. These components are detected by a coil or set of coils to indicate the presence of the marker.
The detection is made difficult because many commonplace objects are magnetic, such as tin cans, keys, shopping trollies, etc. These also have nonlinear characteristics of a greater or lesser degree, and also give rise to varying amounts of the new frequency components.
Many systems of the prior art have used an interrogating magnetic field of a single frequency f.sub.1, and detected a harmonic component n.f.sub.1. In order to discriminate between high-permeability markers and low-permeability common objects, these systems have detected high-order harmonics such as the 20th to 100th harmonic since high permeability materials emit proportionately more at these high orders than common objects. Generally, only the level of the high order harmonic is detected, so the systems are still very prone to false alarm. Some improvement is made by measuring the amount of more than one high order harmonic (usually 2) and confirming the ratio between the two (or more) levels. However, both of these types of system suffer the disadvantage that most of the marker energy is emitted at low harmonic rather than the high orders used for detection, so detectivity is low or else the markers have to be made large, expensive and cumbersome.
A better method exemplified in U.S. Pat. No. 3,990,065 is to use two frequencies, one low f.sub.1, and one high f.sub.2, and to detect an intermodulation product of these two frequencies: f.sub.2 +2f.sub.1. The '065 patent shows use of a third frequency f.sub.3 to scan the interrogation fields around in spatial orientation, but this is not material to the present application. The generation of signal at f.sub.2 +2f.sub.1, is preferential to markers compared to common objects, and furthermore since this is a very low order intermodulation product, it contains a lot of energy for detection. The disadvantage of the '065 method is that once again only a single or narrow-band frequency is detected, so the information content of the signal is low. Furthermore since f.sub.1 is very low compared to f.sub.2, the detected frequency is very close to an emitted frequency f.sub.2, which contains a lot of power, therefore emitter and receiver bandwidth have to be very narrow and carefully defined if the emitter is not to swamp the receiver with background signal. This places severe design constraints on the electronic circuitry.
Another system is shown in EP 0153286 of the present assignee. Here a low frequency f.sub.1 is used, together with two further high frequencies f.sub.2 and f.sub.3. f.sub.2 and f.sub.3 are significantly different from each other, and are emitted from separate coils which are physically separated from each other. Detection is carried out around an intermodulation product frequency n.f.sub.2 +m.f.sub.3 (usually f.sub.2 +f.sub.3) in a frequency band which includes the sidebands of twice the low frequency f.sub.1. This system has the advantage that the detected frequency is very far from any emitted frequency, so the filter design is eased. Furthermore, a large bandwidth around n.f.sub.2 and m.f.sub.3 is available (i.e. free from emitted signal), which is rich in intermodulation information which can be used to distinguish the presence of markers. The disadvantage of this system is the need for two coils, the need for generating three separate frequencies, and the consequent complexity in electronic and mechanical design. Furthermore, even the low order product f.sub.2 +f.sub.3 is not the lowest available intermodulation frequency, so it has limited available energy.